---
layout: docs
page_title: Glossary
description: Nomad key concepts include allocation, authoritative regions, bin packing, client, datacenters, deployment, driver, evaluation, job, node, node pool, regions, server, task, and task group.
---

# Glossary

This glossary provides definitions and explanations for important terms and
concepts used in Nomad.

## Allocation

An Allocation is a mapping between a task group in a job and a client node. A
single job may have hundreds or thousands of task groups, meaning an
equivalent number of allocations must exist to map the work to client
machines. Allocations are created by the Nomad servers as part of scheduling
decisions made during an evaluation.

## Authoritative and Non-Authoritative Regions

The authoritative region is the region in a federated multi-region cluster that
holds the source of true for entities replicated across regions, such as ACL
tokens, policies, and roles, namespaces, and node pools.

All other regions are considered non-authoritative regions and replicate these
entities by pulling them from the authoritative region.

## Bin Packing

Bin Packing is the process of filling bins with items in a way that maximizes
the utilization of bins. This extends to Nomad, where the clients are "bins"
and the items are task groups. Nomad optimizes resources by efficiently bin
packing tasks onto client machines.

## Client

A Nomad client is an agent configured to run and manage tasks using available
compute resources on a machine. The agent is responsible for registering with
the servers, watching for any work to be assigned and executing tasks. The
Nomad agent is a long lived process which interfaces with the servers.

## Datacenters

Nomad models a datacenter as an abstract grouping of clients within a
region. Nomad clients are not required to be in the same datacenter as the
servers they are joined with, but do need to be in the same
region. Datacenters provide a way to express fault tolerance among jobs as
well as isolation of infrastructure.

## Deployment

Deployments are the mechanism by which Nomad rolls out changes to cluster state
in a step-by-step fashion. Deployments are only available for Jobs with the type
`service`. When an Evaluation is processed, the scheduler creates only the
number of Allocations permitted by the [`update`][] block and the current state
of the cluster. The Deployment is used to monitor the health of those
Allocations and emit a new Evaluation for the next step of the update.

## Driver

A Driver represents the basic means of executing your **Tasks**.  Example
Drivers include Docker, QEMU, Java, and static binaries.

## Evaluation

Evaluations are the mechanism by which Nomad makes scheduling decisions.  When
either the _desired state_ (jobs) or _actual state_ (clients) changes, Nomad
creates a new evaluation to determine if any actions must be taken. An
evaluation may result in changes to allocations if necessary.

## Job

A Job is a specification provided by users that declares a workload for
Nomad. A Job is a form of _desired state_; the user is expressing that the
job should be running, but not where it should be run. The responsibility of
Nomad is to make sure the _actual state_ matches the user desired state. A
Job is composed of one or more task groups.

## Node

A more generic term used to refer to machines running Nomad agents in client
mode. Despite being different concepts, you may find `node` being used
interchangeably with [`client`](#client) in some materials and informal
content.

## Node Pool

Node pools are used to group [nodes](#node) and can be used to restrict which
[jobs](#job) are able to place [allocations](#allocation) in a given set of
nodes. Example use cases for node pools include segmenting nodes by environment
(development, staging, production), by department (engineering, finance,
support), or by functionality (databases, ingress proxy, applications).

## Regions

Nomad models infrastructure as regions and datacenters. A region will contain
one or more datacenters. A set of servers joined together will represent a
single region. Servers federate across regions to make Nomad globally aware.

In federated clusters one of the regions must be defined as the [authoritative
region](#authoritative-and-non-authoritative-regions).

## Server

Nomad servers are the brains of the cluster. There is a cluster of servers per
region and they manage all jobs and clients, run evaluations, and create task
allocations.  The servers replicate data between each other and perform leader
election to ensure high availability. More information about latency
requirements for servers can be found in [Network
Topology](/nomad/docs/deploy/production/requirements#network-topology).

## Task

A Task is the smallest unit of work in Nomad. Tasks are executed by drivers,
which allow Nomad to be flexible in the types of tasks it supports. Tasks
specify their driver, configuration for the driver, constraints, and resources
required.

## Task Group

A Task Group is a set of tasks that must be run together. For example, a web
server may require that a log shipping co-process is always running as well. A
task group is the unit of scheduling, meaning the entire group must run on the
same client node and cannot be split.

[`update`]: /nomad/docs/job-specification/update
